The present invention relates in general to a method and apparatus for securing pressure-containing threaded tubular connections in remote or hazardous locations. More particularly, the invention relates to a method and apparatus for using electrical heating to remotely pretension pressure containing threaded tubular connections.
Tubular connections for containing high pressure are widely used by industry, and a very large number of different types have been developed for specific needs. Several types based upon threaded connections are used frequently in oilfield applications. One very common type of connection used for pipes has tapered threads and relies upon interference between the male and female threads when the connection halves are screwed together. The National Pipe Thread (NPT) commonly used in the United States belongs to this first type of connection. This type of connection requires high makeup and breakout torques and liberal lubrication for even small sizes.
A second type of threaded connection utilizes the interference fit of a transverse sealing shoulder on each side of the connection where the annular shoulders are forced together as the threads are made up. This second type of connection can use either integral threads so that the sealing shoulders must be rotated relative to each other or a nut on one side can be used to avoid relative rotation of the sealing shoulders. Standard American Petroleum Institute (API) drill pipe threads are a typical example of a relatively rotating seal of this second type of connection. For large sizes, very high makeup torques and ample lubrication are required. This type of connection can be modified to use an annular face seal in a groove in one of the comating sealing shoulders either as the only seal or as a secondary seal. Certain types of seals such as elastomers or metal C-rings or metal O-rings do not require high compression loads to achieve sealing, but high interfacial preloads may be needed in order to resist high tensile or bending loadings transferred across the connection. Another type of modification for this type of seal uses an intermediate annular seal washer which is compressed between the two transverse sealing faces.
A third type of connection is similar to the second, but uses the interference fit of comating conical faces to achieve sealing. Again, the connection can have either relative rotation or no relative rotation. This type of connection is frequently used in oilfield tubing and casing. The torques to achieve sealing are somewhat reduced, but the hoop stresses adjacent the comating conical surfaces must be controlled by limiting the interference fit. Again, the torques are only somewhat reduced and good lubrication is still required.
A fourth type of screwed connection uses a seal ring in an annular groove on a cylindrical surface of a first connection half to achieve radial sealing between the grooved part and a comating cylindrical surface on the second connection half. The seal can be elastomeric, such as an O-ring or a C-ring, or the seal can be a metal O-ring or some other metal-to-metal seal. This type of connection does not require much lubrication or high makeup torques unless high loadings must be resisted across the connection. The seal for this type of connection can be either rotating or nonrotating relative to its comating surface.
Another type of connection uses flanges having multiple bolts or studs and nuts in a bolt circle pattern to pull the two sides of the connection together and achieve sealing. API or ANSI flanges are examples of this type of connection. Either annular face seals with or without intermediate gaskets or annular wedging interference fit seals such as conical faced ring gaskets are typically used. Lubrication is not as severe a problem as for the first, second, and third connection types. However, even though the torques required to makeup or breakout the fasteners are less than those for a single thread pair joint, they can still be substantial. Further, unpredictable variations of friction makes obtaining uniform bolting preloads unlikely, thereby impairing joint reliability.
The basic problem with all these types of connections when they are used to transmit high loadings as well as seal high pressures is the need to accurately control the makeup torque and friction of the threads so that a predictable value of joint preload can be obtained. This is essential, even for cases where the seal does not require high preload, so that the connection can resist bending moments and the threads of any connections are not subject to large magnitude load cycling with attendant metal fatigue. Unfortunately, such predictability is very hard to obtain, and the predictability becomes much less when the connections are made remotely in a difficult environment, such as sub sea. When the connection must be repeatedly made up and broken out remotely, joint torque predictability and, hence, connection and seal reliability become very problematic. For such situations, thread galling, seal damage, misalignment, and the need for very high torques further complicate the problem.
What is urgently needed is a new type of high pressure, high load connection which can be made up and broken out repeatedly without significant risk of failure and without the need for very high torques. Avoidance of seal damage, thread damage, alignment problems, reduced sensitivity to lubrication problems, and general improvements in reliability are strongly needed. Improvements in connection behavior predictability will result if these needs are met. These requirements become particularly critical for remote connections where general robustness of the connection is essential and the connection must be adaptable to widely varying and difficult installation conditions with only limited informational feedback to the installer.
The invention contemplates a simple, inexpensive device for solving the problems and disadvantages of the prior approaches discussed above. The present invention provides a rapid, reliable and accurate pretensioning of threaded tubular connections in remote locations. The invention is a method and apparatus for making a pressure-containing threaded tubular connection, suitable for remotely operated connection and separation, utilizing electrical heating to reduce the amount of makeup torque required to produce reliable preloading of the connection.
One aspect of the present invention comprises: (1) a first hub having a through bore, an interior end and an external surface, wherein the first hub has a plurality of threads on a portion of the external surface; (2) a second hub having a through bore and a first end; (3) a coupling nut coaxially mounted on the second hub to limit axial movement of the coupling nut along a length of the second hub, wherein the coupling nut has a plurality of internal threads adapted to threadedly engage the threads on the external surface of the first hub, wherein tightening the coupling nut forces the interior end of the first hub towards the first end of the second hub; and (4) heating means for heating the coupling nut; whereby when the coupling nut is heat-expanded less torque is required to tighten or loosen the coupling nut.
Another aspect of the present invention comprises: (1) a first hub having a through bore, a first end and an externally threaded exterior section proximal the first end; (2) a second hub having a through bore, an outer surface, an interior end, and a shoulder opposed to the interior end; (3) a coupling nut having an internally threaded proximal end adapted to threadedly engage the externally threaded exterior section of the first hub, and an internal shoulder distal to the threaded proximal end engaged with the shoulder of the second hub to provide axial and radial position control for said coupling nut, wherein tightening the coupling nut forces the first end of the first hub towards the interior end of the second hub; a heating element attached to said coupling nut; and (4) torquing means for transmitting torque to the coupling nut to tighten or loosen the nut; whereby when the heating element increases the temperature of the coupling nut sufficiently the coupling nut is expanded and less torque is needed to achieve a predetermined pretensioning of the tubular connector.
Yet another aspect of the present invention is a method of prestressing a threaded connection comprising the steps of:
(1) selecting a tubular connector to connect two lengths of tubing, the connector comprising: a first hub having a through bore, an interior end and an external surface, wherein the first hub has a plurality of external threads on a portion of the external surface; a second hub having a through bore and a first end; a coupling nut coaxially mounted on the second hub to limit axial movement of the coupling nut along a length of the second hub, wherein the coupling nut has a plurality of internal threads adapted to threadedly engage the threads on the external surface of the first hub, wherein tightening the coupling nut forces the interior end of the first hub towards the first end of the second hub; and a heating element for heating the coupling nut;
(2) aligning the internal threads of the coupling nut for engagement with the external threads of the first hub;
(3) applying an initial torque to the coupling nut to engage the external and internal threads to form a connection between the first hub and the coupling nut with a first level of tension on the connection;
(4) thermally expanding the coupling nut;
(5) applying a secondary torque to the coupling nut to provide a second level of tension on the connection; and
(6) allowing the coupling nut to achieve ambient temperature to achieve a third level of tension on the connection.
The foregoing has outlined rather broadly several aspects of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and the specific embodiment disclosed might be readily utilized as a basis for modifying or redesigning the structures for carrying out the same purposes as the invention. It should be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims.